UNIVERSITY  OF  CALIFORNIA  agricultural  Experiment  Station 

COLLEGE  OF  AGRICULTURE  E.   J.   WlCKSON,   Director 

BERKELEY,  CALIFORNIA 

CIRCULAR  No.  75. 

February,  1912. 


FUMIGATION  STUDIES  No.  6. 

A  NEW  LEAKAGE  GAUGE 

By  C.  W.  Wood  worth. 

We  have  pointed  out  in  several  previous  bulletins  of  this  series  the 
great  importance  of  leakage  in  the  determination  of  the  correct  dose  in 
fumigating,  but  there  has  not  been  hitherto  any  practical  method  of 
determining  the  leakage.  A  new  method  has  now  been  perfected  by 
the  use  of  a  simple  piece  of  apparatus,  to  be  described  in  the  present 
circular,  whereby  this  determination  can  be  made  very  quickly  and 
accurately. 

The  apparatus  is  shown  in  Figs.  1  and  2.  It  consists  of  a  clamp  (d) 
by  means  of  which  a  portion  of  the  tent  to  be  tested  is  firmly  held  in 
the  apparatus  and  a  set  of  three  tubes,  one  of  brass,  ending  in  a  nipple 
(i)  for  attaching  a  rubber  mouthpiece,  and  the  other  two  of  glass 
(b  and  c)  along  one  of  which  is  a  scale  so  graduated  as  to  show  the 
degree  of  leakage.  All  these  tubes  communicate  with  a  small  glass 
chamber  filled  with  water. 

The  method  of  determining  the  leakage  is  as  follows : 

1.  Unscrew  the  glass  chamber  (a)  filling  the  glass  part  full  of  water, 
then  replace  it  on  the  instrument.  If  properly  filled,  the  water  will  not 
be  seen  in  the  glass  tubes  above  the  top  of  this  chamber. 

2.  Clamp  a  fold  of  the  tent  to  be  tested  in  the  instrument.  The 
reason  for  testing  a  double  thickness  of  the  tent  is  that  this  enables  one 
to  test  any  part  of  the  tent  with  equal  facility.  The  gradations  are 
planned  for  double  tents  and  would  not  indicate  the  correct  leakage  for 
a  single  thickness. 

3.  Blow  gently  through  the  rubber  tube  till  the  water  is  seen  to  rise 
in  one  or  both  of  the  glass  tubes,  blowing  hard  enough  to  bring  the 
water  to  the  top  of  one  of  them,  and  read  the  degree  of  leakage  on  the 
scale  where  the  top  of  the  column  of  water  reaches  in  the  other  tube. 

The  earlier  studies  of  the  processes  of  fumigation  are  as  follows  : 

1.  Orchard  Fumigation,  Bulletin  126. 

2.  Fumigation  Dosage,  Bulletin  152. 

3.  Fumigation  Practice,  Circular  11. 

4.  Fumigation  Schedule,   Circular  50. 

5.  Dosage  Tables,  Bulletin  220. 


Fig.  1. — The  new  leakage  gauge,  a,  water  reservoir ;  b,  gauge  tube  with  water 
column  at  "test"  height;  c,  open  gauge  tube  showng  water  column  at  top  ;  d,  clamp 
lever ;  e,  outer  clamp  spring ;  f,  inner  clamp  spring ;  g,  clamp  ring ;  h,  rubber 
washer ;  i,  nipple  of  brass  tube ;  j,  opening  to  open  gauge  tube ;  k,  passage  to 
closed  gauge  tube  ;   I,  passage  to  brass  tube  ;  m,  washer  of  reservoir. 


40 


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20 


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*""":""["  r"J'rri. 

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u 

4.  The  dose  of  cyanide,  which  should  be  used 
to  correspond  with  the  degree  of  leakage  found 
by  this  apparatus,  is  obtained  by  the  methods 
given  in  detail  in  Bulletin  220. 

THEORY  OF  LEAKAGE  GAUGES. 

While  the  above  directions  are  all  that  is 
necessary  to  understand  in  order  to  obtain 
accurate  dosage,  it  is  desirable  that  the  user 
should  understand  the  principles  upon  which 
the  apparatus  is  based  and  the  details  of  its 
construction. 

The  theory  upon  which  all  methods  of  meas- 
uring leakage  is  based  is,  that  the  rates  of 
leakage  of  gas  through  different  pieces  of  cloth 
will  show  approximately  the  same  difference 
that  is  found  between  the  rates  of  flow  of  air 
through  these  cloths,  when  it  is  forced  through 
under  the  same  pressure ;  thus,  if  under  identi- 
cal conditions  one  litre  of  air  will  pass  through 
one  piece  of  canvas  while  two  litres  pass 
through  another,  the  second  is  assumed  to  per- 
mit th  escape  of  the  cyanide  twice  as  rapidly 
as  the  former. 

There  are  two  practical  methods  of  deter- 
mining the  rate  at  which  air  will  flow  through  a 
piece  of  cloth.  The  one  first  used  in  our  experi- 
ments, and  the  only  one  used  by  other  experi- 
menters, was  to  measure  the  volume  of  air  pass- 
ing through  in  a  given  time,  or  more  often  the 
time  required  for  a  definite  volume  to  pass,  the 
pressure  forcing  the  air  through  the  cloth,  of 
course,  must  be  maintained  uniform  in  making 
such  determinations.  The  method  utilized  in 
the  apparatus  here  described  is,  to  measure  the 
resistance  to  the  passage  of  the  air  presented 
by  the  cloth  to  be  tested  as  compared  with  an 
opening  of  a  known  size.  The  particular  ad- 
vantage of  this  second  method  is  the  simplicity 
of  the  apparatus  and  the  rapidity  of  the  deter- 
minations. 

METHOD    OF   STATING   LEAKAGE. 


Fig.   2. — The  new  leakage 


Heretofore    the    relative    leakage    has    been 

ated  in  terms  of  quantity  of  air  per  minute  or 

Figerlns   the   Same   aS   °f  minutes  per  given  quantity  and  must  be  inter- 


fn^graduatXnJ  ^The   stated  in  terms  of  quantity  of  air  per  minute  or 


—  4  — 

preted  by  noting  the  size  of  the  area  tested  and  the  pressure.  A  very 
much  more  rational  method  is  by  the  use  of  percentages.  The  method 
of  determining  the  per  cent  of  leakage  is  as  follows:  The  apparatus 
used  in  measuring  the  leakage  in  our  first  experiments  consisted  of  a 
vessel  holding  a  litre  of  water,  closed  except  for  two  openings,  a  larger 
one  below  for  the  escape  of  the  water  and  one  above  for  admission  of 
air.  Over  this  latter  opening  was  clamped  a  sample  of  the  cloth  to  be 
tested.  The  water  would  run  out  as  fast  as  the  air  could  flow  through 
the  section  of  the  tent  being  tested  to  replace  the  water.  The  time  is 
determined  by  the  use  of  a  stop  watch.  By  allowing  the  air  to  enter 
through  a  small  hole  of  a  known  size,  instead  of  through  the  cloth,  the 
time  required  for  a  litre  of  air  to  pass  could  be  similarly  determined. 
This  was  done  repeatedly  during  the  course  of  our  experiments  with 
holes  of  various  sizes,  and  in  this  way  a  formula  was  developed  for  cal- 
culating the  rate  of  flow  through  holes  of  any  desired  size.  It  is  then 
but  a  simple  matter  to  calculate  the  ratio  between  the  area  of  the  sample 
of  the  canvas  held  in  the  clamp  for  testing,  and  of  a  hole  of  a  definite 
size  to  obtain  the  percentage.  In  the  average  fumigation  tent,  according 
to  our  determinations,  an  equivalent  hole  was  found  to  be  one  four 
hundredths  part  of  the  total  area  of  the  sample,  giving  a  leakage  there- 
fore of  one  quarter  of  one  per  cent.  The  leakage  through  a  piece  of 
tent  cloth  two  inches  square  (4  sq.  in.)  would  in  an  average  fumigating 
tent  be  equivalent  to  a  single  hole  one  tenth  of  an  inch  square.  The 
total  amount  of  leakage  will  depend  upon  the  leakage  surface,  and  the 
same  tent  will  give  a  varying  amount  of  leakage  whether  on  a.  large 
or  a  small  tree.  The  proportion  of  gas  that  escapes  from  a  tent  will 
vary  also  with  the  size  of  the  tree  covered,  since  the  large  space  contains 
a  greater  volume  within  each  unit  of  surface.  The  percentage  of  leak- 
age corresponds  with  total  leakage  or  with  the  proportionate  loss  of  gas 
only  when  the  volumes  are  equal.  The  same  tent  will  vary  according 
to  the  conditions  of  the  weather.  When  the  fibres  are  swollen  by  ab- 
sorbing moisture  the  spaces  are  of  necessity  smaller  and  the  leakage 
is  correspondingly  diminished,  on  the  other  hand  when  the  air  is  very 
dry  the  fibres  shrink  leaving  larger  interspaces  and  the  leakage  becomes 
greater.  Thus,  the  same  tent  may  show  a  different  per  cent  of  leakage 
at  different  times. 

It  will  be  necessary  to  determine  by  means  of  the  leakage  gauge  the 
exact  amount  of  this  change  and  then  from  the  proper  dosage  table, 
given  in  Bulletin  220,  one  can  determine  the  correct  dose,  thus  insuring 
uniform  strength  of  gas  within  the  tent  to  the  end  of  the  killing  period. 

LEAKAGE    IN    TORN    TENTS. 

We  are  now  able  for  the  first  time  to  estimate  the  effect  of  working 
with  a  tent  which  has  been  snagged  or  torn,  and  compare  it  with  the 
changes  in  leakage  due  to  the  character  of  the  cloth  or  to  the  condition 


-  5  - 

of  the  weather.  It  is  entirely  possible  for  a  tent  to  be  full  of  conspicu- 
ous holes  and  still  be  tighter  than  a  new  tent  with  a  looser  weave  of 
canvas. 

The  leakage  gauge  is  marked  off  into  divisions  corresponding  with 
0.05  per  cent  of  leakage.  The  following  table  shows  the  number  of 
square  inches  corresponding  with  this  amount  of  leakage  in  tents  of 
various  sizes: 

Table  I. 

Size  of  holes  corresponding  to  0.05  per  cent  leakage. 
Size.  Leakage   area. 

17  feet 17  square  inches. 

24  feet 33  square  inches. 

30  feet 52  square  inches. 

36  feet 75  square  inches. 

41  feet 87  square  inches. 

43  feet : 107  square  inches. 

45  feet 117  square  inches. 

48  feet 133  square  inches. 

52  feet 156  square  inches. 

55  feet 175  square  inches. 

64  feet 237  square  inches. 

72  feet 300  square  inches. 

The  aggregate  of  107  square  inches  of  holes  in  a  tent  43  feet  across 
allows  a  leakage  of  only  0.05  per  cent  or  only  a  fifth  of  the  average 
degree  of  leakage  due  to  the  weave  of  the  cloth. 

Fumigators  are  very  watchful,  and  properly  so,  for  holes  in  the  tent, 
but  ignore  the  variations  of  leakage  through  the  weave  of  the  cloth, 
which  may  often  be  of  very  much  greater  importance. 

In  the  case  of  a  43-foot  tent  the  average  leakage  is  equivalent  to 
holes  having  an  aggregate  area  of  535  square  inches  (5  times  107).  The 
difference  of  leakage  on  a  very  dry  night  will  equal  at  least  214  square 
inches  additional,  or  a  total  of  849  square  inches.  Any  inspector 
would  condemn  a  tent  with  half  a  dozen  tears  big  enough  to  put  one's 
fist  through,  while  their  average  area  might  not  exceed  three  square 
inches,  or  a  total  increased  leakage  not  over  one  tenth  of  the  214  square 
inches  of  additional  leakage  due  to  dryness. 

There  is  no  doubt  of  the  importance  of  looking  for  and  mending  the 
tears  of  a  fumigating  tent,  but  it  is  more  important,  and  very  much 
more  important,  to  measure  the  leakage  of  the  cloth. 

In  the  above  table  we  have  supposed  that  the  holes  were  evenly 
distributed,  but  since  they  usually  occur  in  the  center  portion  of  the 
tent  rather  than  in  the  skirt,  a  more  satisfactory  manner  of  estimating 
its  amount  is  by  figuring  the  portion  of  the  tent  off  the  ground,  as  is 
done  in  Table  II.  The  amount  in  this  case  varies  with  the  size  of  the 
tree  as  measured  by  the  distance  over  the  top.  Some  of  the  leakage 
may  be  decreased  by  the  folding  of  the  cloth  down  the  sides,  and  there- 
fore the  leakage  of  a  tent  made  just  to  fit  the  tallest  shaped  trees 
(bell-shaped)  is  given  to  show  the  smallest  possible  leakage.  A  bell 
tent  on  a  low,  broad  tree  would  be  much  more  like  a  sheet  tent. 


In  any  case,  a  greater  leakage  can  be  offset  by  increasing  the  dose. 

If,  for  instance,  a  tent  on  a  tree  measuring  30  feet  over  the  top  which 

showed  a  leakage  of  0.2  per  cent  by  the  gauge,  but  which  had  one  or 

more  tears  amounting  to  about  51  spuare  inches  in  area,  one  should 

use  the  0.25  per  cent  table  for  determining  the  dose.     The  same  tent 

on  a  previous  night  before  it  was  torn  might  have  shown  0.3  per  cent 

leakage,  and  required  the  use  of  the  still  larger  dose  given  in  the  0.3 

per  cent  table. 

Table  II. 

Size  of  holes  corresponding  to  0.05  per  cent  leakage. 
Distance  over.  Bell  tent.  Sheet  tent. 

10  feet 2.95  square  inches.  5.66  square  inches. 

12  feet 4.24  square  inches.  8.16  square  inches. 

14  feet 5.78  square  inches.  11.10  square  inches. 

16  feet 7.54  square  inches.  14.46  square  inches. 

18  feet 9.54  square  inches.  18.33  square  inches. 

20  feet 11.80  square  inches.  22.65  square  inches. 

22  feet 14.25  square  inches.  27.40  square  inches. 

24  feet 16.95  square  inches.  32.60  square  inches. 

26  feet 19.90  square  inches.  38.30  square  inches. 

28  feet 23.10  square  inches.  44.40  square  inches. 

30  feet 26.55  square  inches.  51.00  square  inches. 

32  feet 30.20  square  inches.  58.00  square  inches. 

34  feet 34.50  square  inches.  65.40  square  inches. 

36  feet 38.20  square  inches.  73.40  square  inches. 

38  feet 42.60  square  inches.  81.60  square  inches. 

40  feet 47.20  square  inches.  90.60  square  inches. 

42  feet ,.  52.00  square  inches.  99.80  square  inches. 

•  44  feet 57.00  square  inches.  109.50  square  inches. 

46  feet 62.20  square  inches.  119.50  square  inches. 

48  feet 67.90  square  inches.  130.40  square  inches. 

50  feet 73.60  square  inches.  141.30  square  inches. 

52  feet 79.60  square  inches.  152.80  square  inches. 

54  feet 85.90  square  inches.  165.00  square  inches. 

56  feet 92.40  square  inches.  177.50  square  inches. 

58  feet 99.20  square  inches.  190.50  square  inches. 

60  feet 106.00  square  inches.  203.70  square  inches. 

62  feet 113.30  square  inches.  217.60  square  inches. 

THE  TENT  CLAMP. 

In  our  first  experimental  work  we  used  the  method  which  has  also 
been  employed  by  others,  of  tying  the  sample  to  be  tested  over  the  end 
of  a  pipe.  We  soon  appreciated,  however,  that  for  practical  use  in  the 
field  we  needed  a  clamp  that  could  be  instantly  applied  and  experi- 
mented with  a  number  of  forms. 

The  first  satisfactory  form  of  clamp  is  shown  in  Fig.  3.  It  consists 
of  two  cups  (d  and  e)  held  together  by  a  strong  spring  (a)  and  opened 
by  a  pair  of  arms  (6  and  c)  acting  as  levers,  pulling  the  cups  apart  to 
insert  or  remove  the  cloth  being  tested. 

The  cups  are  attached  loosely  to  the  arms  and  adjust  themselves 
when  closing  over  the  cloth.  The  cavity  of  one  cup  is  smaller  than 
that  of  the  other,  and  the  rim  of  the  larger  rests  against  the  rounded 
outer  edge  of  the  other,  stretching  the  canvas  tightly  against  the  rim 
of  the  smaller  cavity.     The  smaller  cavity  connects  with  a  nipple   (/) 


—  7 


and  the  larger  one  is  open  beneath.  In  operating,  the  nipple  gives 
attachment  by  means  of  a  rubber  tube  to  the  other  part  of  the  appa- 
ratus, and  air  can  be  drawn  or  blown  through  the  portion  of  the  cloth 
held  in  the  clamp. v 

In  designing  this  clamp  I  am  in- 
debted to  Mr.  E.  J.  Hoff,  mechanician 
of  the  U.  S.  Irrigation  Investigation, 
for  the  working  out  of  many  of  the 
details,  and  I  have  had  his  very  effi- 
cient cooperation  in  all  the  mechanical 
work  connected  with  this  study. 

The  form  of  clamp  finally  adopted 
consists  of  a  cup  of  metal  large  enough 
to  hold  an  ordinary  rubber  hose 
washer  (Fig.  lh)  having  an  inside 
diameter  of  five  eighths  of  an  inch,  or 
an  area  of  nearly  .4  of  a  square  inch. 
Over  this  projects  the  end  of  a  strip 
of  brass  (/)  bearing  an  annular  exten- 
sion (g)  on  its  inner  face,  correspond- 
ing to  the  inner  edge  of  the  rubber 
washer.  The  portion  of  the  strip 
within  this  annular  extension  is  cut 
away.  When  a  piece  of  cloth  is  in- 
serted between  the  rubber  washer  and 
this  strip,  and  the  latter  clamped 
down  upon  it,  air  may  be  forced  into  or 
from  the  cup  through  the  area  of  the 
cloth  within  the  ring  of  the  clamp 
approximately  four  tenths  of  a  square 
inch. 

A  second  spring  of  spring  brass  (e) 
ending  exactly  opposite  the  center  of 
the  cup  is  so  bent  as  to  bear  against 
the  first  strip.  Above  this  second 
strip  a  lever  (d)  is  arranged  to  exert 
the  necessary  pressure.  These  two 
strips  are  so  bent  that  the  clamp  is 
equally  tight  on  all  sides.  This  can  be 
determined  by  placing  a  thin  piece  of 
paper  over  a  folded  piece  of  cloth  in  the  clamp  and  noting,  when  remov- 
ing it,  if  the  paper  is  creased  equally  strongly  at  all  points  in  the  circle. 
If  the  lever  were  directly  upon  the  first  strip,  it  could  only  be  adjusted 
for  cloth  of  uniform  thickness,  and  if  the  cloth  were  thicker  than  that 


Fig.  3. — Tent  clamp,  a,  spring  ;  b 
and  c,  levers  to  open  clamp  ;  d 
and   e3   clamp    cups ;    f   nipple. 


—  8  — 

for  which  the  instrument  was  adjusted,  it  would  be  gripped  more 
tightly  on  the  side  nearest  the  clamping  lever,  and  if  thinner,  it 
would  be  tighter  on  the  opposite  side,  but,  with  the  condition  described 
above,  the  clamp  works  with  all  thicknesses  of  canvas  without  an  appre- 
ciable difference  of  compression  on  either  side. 

The  clamp  first  described  remains  normally  closed  by  the  action  of 
the  spring  and  needs  no  attention  while  making  the  test.  The  latter 
clamp  remains  open  or  closed  according  to  the  position  of  the  lever, 
leaving  both  hands  free,  both  while  adjusting  the  cloth  and  while 
determining  the  leakage. 

THE   NEW   PRESSURE   GAUGES. 

The  clamps  described  above  may  be  used  for  either  method  of  deter- 
mining the  leakage.  The  determination  of  leakage  by  the  volume 
method  requires  the  use  of  rather  bulky  apparatus  to  provide  for  the 
displacement  of  a  sufficient  volume  of  air,  and  also  the  use  of  sufficient 
time  to  accurately  gauge  the  difference  between  leakages,  and  means 
for  maintaining  uniform  pressure  during  that  time. 

For  these  reasons  an  entirely  new  principle  was  adopted  as  a  sub- 
stitute for  the  "volume  gauges,"  furnishing  a  class  of  apparatus  which 
may  be  called  ' '  pressure  gauges. ' ' 

If  air  is  forced  through  a  tube  narrowed  at  intervals  by  equal  con- 
strictions producing  a  series  of  chambers,  A,  B,  C,  D,  etc.,  it  is  evident 
that  an  equilibrium  will  be  quickly  secured  in  which  the  quantity  and 
the  velocity  of  the  air  passing  through  each  of  these  constrictions  will 
be  equal.  Since  the  openings  are  all  of  the  same  size,  this  equality  can 
only  occur  when  the  difference  in  the  pressure  of  the  air  in  the  suc- 
ceeding chambers  is  equal.  Thus,  if  the  pressure  in  A  is  ten  pounds  to 
the  square  inch  and  that  in  B  is  9  pounds,  then  the  pressure  in  C  is 
8  pounds,  in  D  7  pounds,  etc.,  giving  a  difference  of  one  pound. 

If  the  constrictions  are  not  of  uniform  size,  since  the  same  quantity 
must  still  pass  through  all  the  openings,  it  is  clear  that  it  must  pass 
more  rapidly  through  the  smaller  openings;  or,  in  other  words,  the 
difference  of  pressure  on  the  two  sides  of  the  smaller  opening  must  be 
larger.  If,  for  instance,  the  constriction  between  B  and  C  is  larger 
than  through  the  others  and  that  between  C  and  D  smaller,  the  pressures 
in  the  various  chambers  might  be  10,  9,  8J,  and  7,  showing  only  half 
a  pound  between  B  and  C  and  a  pound  and  a  half  between  C  and  D. 

If  the  size  of  one  of  the  constrictions  is  known  in  such  a  series  and 
we  have  the  means  of  measuring  the  pressure  in  the  various  chambers, 
it  will  be  rather  a  simple  mathematical  calculation  to  determine  the 
size  of  all  the  other  constrictions. 

The  pressure  gauges  are  based  on  these  well  known  physical  laws 
and  can  be  constructed  with  any  degree  of  sensitiveness  and  in  various 


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10 


Fig.  4. — Test  tube  gauge,  a  and  b,  pressure  chambers  ;  c  and  d,  gauge  tubes  ;  e,  nipple 
leading  to  mouthpiece  ;  f,  hippie  leading  to  tent  clamp  ;  g,  air  column  at  end  of 
gauge  tube  ;  h,  air  column  showing  .24  per  cent  leakage. 


-  10- 

forms.  We  will  describe  two  used  in  our  experimental  work  as  well  as 
the  one  recommended  for  practical  field  uses,  though  several  other  forms 
were  tried  and  discarded  for  one  reason  or  another. 

TEST    TUBE    GAUGE. 

A  form  of  leakage  gauge  found  very  satisfactory  in  practice  which 
was  of  about  the  simplest  possible  construction  (Fig.  4)  consists  of  two 
chambers  (a  and  b)  connected  by  a  very  small  hole,  each  provided  with 
two  tubes,  one  from  each  (c  and  d)  dipping  deep  into  water  contained 
in  a  test  tube  and  the  others  (e  and  /)  leading,  one  to  the  tent  clamp 
and  the  other  to  the  mouth  of  the  operator.  After  attaching  the  clamp 
to  the  tent  one  blows  through  the  instrument,  and  the  pressure  of  the 
air  in  the  two  chambers  is  shown  by  the  length  of  the  air  column  in  the 
tubes  in  the  water  in  the  test  tube. 


Fig.  5. — Operation  of  tent  clamp  and  test  tube  gauge. 

The  amount  of  pressure  to  be  used  is  just  sufficient  to  force  the  air 
to  the  end  of  one  tube  (g)  and  a  scale  is  provided  whereby  the  amount 
of  leakage  in  per  cents  may  be  read  directly  from  the  position  of  the 
air  in  the  other  tube  (h).  The  tubes  extending  into  the  water  must  be 
of  glass  in  order  to  see  the  position  of  the  air  columns.  The  scale  is 
made  on  paper  and  fastened  ''face  to"  on  the  outer  surface  of  the  test 
tube.  When  the  test  tube  is  filled  with  water  the  scale  is  very  easily 
read.  The  chief  precaution  in  using  this  form  of  gauge  is  to  always 
fill  the  test  tube  to  the  same  level. 

THE   SUCTION   GAUGE. 

Exactly  the  same  apparatus  was  used  in  the  laboratory  with  the 
exception  that  a  vacuum  air  pump  was  used  instead  of  blowing  through 
the  apparatus  and  instead  of  dipping  the  glass  tubes  deeply  into  water, 


—  11  — 

only  the  ends  were  beneath  the  surface  (Fig.  6).  The  suction  from  an 
air  pump  drew  air  through  the  cloth  in  the  clamp  and  lifted  the  water 
up  the  glass  tubes  instead  of  forcing  the  air  down.  Also,  the  tubes  were 
made  much  longer  so  that  readings  could  be  made  to  hundredths  of  a 
per  cent.  This  plan  is  more  accurate  also,  because  a  large  dish  of  water 
may  be  used,  and  the  level  will  not  change  appreciably  with  much  or 
little  leakage. 

The  amount  of  suction  was  just  sufficient  to  raise  the  water  column 
in  one  tube  to  the  zero  point  and  the  per  cent  of  leakage  read  on  a  scale 
along  the  other  column. 

THE   REGULATING   CLAMP. 

It  was  necessary  to  construct  a  clamp  whereby  the  amount  of  suction 
in  the  suction  gauge  could  be  regulated  very  accurately.  The  clamp 
shown  in  Fig.  7  gives  perfect  control  of  the  suction.  It  consists  of  two 
pieces  of  half  inch  strap  iron,  with  a  screw  at  each  end  of  the  lower 
piece  (6)  fitting  loosely  in  the  upper  piece  (a).  One  screw  is  long, 
and  has  a  clock  wheel  soldered  to  the  head  (e).  Along  the  center  of 
the  lower  face  of  the  upper  piece  a  short  piece  of  wire  (c)  is  soldered 


33" 


Fig.  6. — Operaton  of  suction  gauge  testing  a  sample  of  tent  material. 

near  the  end  containing  the  short  screw  (d).     The  rubber  hose  (f)  is 
compressed  beneath  this  wire. 

The  clamp  is  adjusted  approximately  by  means  of  the  small  screw, 
using  a  screwdriver,  and  then  accurately  by  the  large  screw,  which 
works  by  hand.  When  ready  to  operate  the  test  the  hand  screw  is 
turned  down,  completely  closing  the  rubber  suction  tube,  then  the  cloth 
is  placed  in  the  tent  clamp  and  the  screw  turned  up  slowly  until  the 
water  column  in  one  tube  reaches  the  zero  point.  The  column  can  be 
held  perfectly  steady  at  this  point  while  the  reading  is  being  taken  on 
the  scale  along  the  other  column. 

GRADUATING  THE  SCALE. 

When  there  is  no  leakage  both  columns  will  move  in  an  identical 
manner  and  both  will  reach  the  point  selected  as  the  zero  point.  With 
100  per  cent  leakage,  that  is,  with  nothing  in  the  tent  clamp,  only  one 
column  will  move  at  all,  the  surface  of  the  water  being  the  100  per  cent. 
Between  0  and  100  per  cent  the  scale  can  be  most  easily  made  by  the 
use  of  a  curve  constructed  in  the  manner  described  below. 


-  12  — 

1.  Draw  a  line  as  long  as  the  distance  from  the  0  to  the  100  per  cent 
point,  at  the  100  per  cent  end  draw  a  base  line  perpendicular  to  the 
other. 

2.  Place  a  disc  in  the  tent  clamp  having  a  hole  of  a  known  size  and 
find  the  point  on  the  gauge  tube  which  indicates  the  resulting  leakage. 
Plat  this  point  on  the  line  being  graduated. 

3.  Calculate  the  area  of  the  hole  in  the  disc  and  divide  this  amount 
by  the  area  of  the  cup  of  the  tent  clamp.  Draw  a  line  from  the  leakage 
point  previously  determined  parallel  with  the  base  line,  and  lay  off 


Fig.  7. — Regulating  clamp,     a,  upper  bar  ;  h,  lower  bar  ;  c,  compression  blade  ;  d.  coarse 
adjusting  screw  ;  e,  fine  adjusting  screw  ;  f,  rubber  tube  being  compressed. 

along  this  line  a  distance  corresponding  with  the  per  cent  of  leakage 
area  just  calculated. 

4.  Repeat  items  three  and  four  with  at  least  one  other  hole  of  a 
different  size. 

5.  Draw  a  curve  beginning  at  the  zero  point  and  passing  through  the 
points  just  laid  off  and  approaching  the  base  line,  but  not  actually 
reaching  it. 


—  13  — 

6.  Draw  a  series  of  lines  at  right  angles  to  the  base  line  at  distances 
corresponding  to  hundredths  of  a  per  cent  for  instruments  with  long 
columns,  or,  .05  per  cent  for  the  ordinary  short  field  gauges. 

7.  Where  these  per  cent  lines  intersect  the  curve,  draw  lines  parallel 
to  the  base  line,  intersecting  the  first  line  drawn,  and  these  are  the 
gradation  points  for  the  instrument. 

Fig.  8  shows  the  gradations  used  in  the  suction  gauge  and  illustrates 
the  method  of  graduation  described  above. 

The  instrument  should  be  so  constructed  that  the  ratio  of  area  between 
the  constriction  and  the  cup  of  the  tent  clamp  should  correspond 
approximately  with  the  average  leakage  of  the  tents  to  be  tested.  Since 
we  desire  to  test  this  cloth  doubled,  allowance  must  be  made  for  this 
fact,  and  the  area  of  the  tent  clamp  should  be  about  700  times  that  of 
the  constriction  or  26£  times  as  large  in  diameter.  The  drill  used  for 
the  small  hole  in  the  instruments  used  in  this  study  was  .6  mm.  in 
diameter.  This  will  bring  the  readings  of  the  commonest  leakage  near 
the  center  of  the  scale. 

THE  COMBINED  APPARATUS. 

The  form  of  leakage  gauge  which  we  are  recommending  combines  the 
tent  clamp  and  the  gauge  in  one  machine  (Figs.  1  and  2).  The  struc- 
ture of  the  clamp  has  already  been  described  above.  The  gauge  differs 
from  either  of  the  forms  just  described  by  the  fact  that  the  pressure 
from  the  first  chamber  is  communicated  to  the  surface  of  the  water 
reservoir  which  would  drive  the  water  up  both  gauge  tubes  alike,  except 
for  the  back  pressure  in  one  of  the  gauge  tubes. 

The  reservoir  (a)  which  is  a  glass  or  metal  cup,  is  screwed  fast  to  the 
end  of  the  instrument  and  into  it  extends  the  two  glass  gauge  tubes. 
A  brass  tube  (i)  to  which  the  rubber  mouthpiece  is  attached  also  opens 
into  this  chamber,  but  does  not  dip  beneath  the  water.  One  of  the  glass 
tubes  is  open  above  to  the  outside  air  (at  j),  the  other  opens  into  the 
cup  of  the  tent  clamp  (at  h).  This  brass  tube  connects  by  a  very  small 
hole  (I)  with  the  cup  of  the  tent  clamp  which  forms  the  second  chamber 
of  the  apparatus. 

THE  TESTER. 

The  leakage  gauges  are  not  liable  to  get  out  of  order,  but  if  dirt  or 
water  gets  into  the  small  hole  connecting  the  two  chambers,  the  results 
will  be  too  high,  that  is,  the  tents  will  appear  to  be  more  leaky  than 
they  really  are. 

A  small  disc  of  brass  should  therefore  accompany  every  instrument 
to  use  as  a  tester,  and  a  special  line  on  the  scale  will  show  the  point  at 
which  the  water  should  stand  in  the  gauge  tube  when  the  tester  is 
placed  in  the  clamp.  The  hole  in  the  tester  should  be  clean  since  the 
presence  of  any  dirt  will  cause  the  column  of  water  to  stand  nearer 
the  zero  point. 


0°/< 


Test 


Test 


Fig.   8. — Scale  showing  method  of  gradation. 


—  15  — 

Either  of  these  holes  may  be  cleaned  by  using  a  wooden  toothpick, 
but  no  metal  should  be  used  for  fear  of  permanently  enlarging  the 
holes.  The  accuracy  of  the  instrument  depends  upon  the  small  hole 
connecting  the  chamber  remaining  the  same  size.  If  for  any  reason  the 
instrument  will  not  give  the  correct  reading  with  the  tester,  it  may  be 
sent  to  the  maker  for  adjustment. 

r£sum£. 

This  Circular  gives  directions  for  using  a  leakage  gauge  on  fumiga- 
tion tents. 

The  theory  of  leakage  determination  is  explained. 

The  use  of  per  cents  to  express  the  leakage  of  tents  is  the  most  rational 
method. 

Leakage  through  the  weave  of  the  cloth  is  much  more  important  than 
that  through  visible  holes,  and  the  necessary  adjustment  of  the  dose  for 
either  is  illustrated. 

Different  forms  of  tent  clamps  are  described. 

Pressure  gauges  for  determining  leakage  are  more  practical  than 
volume  gauges. 

A  test  tube  gauge  is  the  simplest  form  of  pressure  gauge. 

A  suction  gauge  is  the  form  most  satisfactory  in  the  laboratory  and 
requires  a  special  form  of  regulating  clamp. 

The  method  of  graduating  leakage  gauges  is  explained. 

A  combined  instrument,  containing  both  tent  clamp  and  gauge,  is 
recommended  for  field  work. 

The  use  of  a  tester  enables  one  to  be  sure  of  the  proper  adjustment 
of  the  instrument. 


